Synaptic plasticity plays a fundamental role in the function of neural circuits. However, if potentiation mechanisms reinforce effective synapses, then positive feedback could lead to a saturation of response gain across inputs. Without homeostatic mechanisms to down-scale synaptic strengths while maintaining balanced excitation and inhibition, neural networks would cease functioning. Additionally, synaptic homeostasis may play an etiological role in disorders where the balance of excitation and inhibition is altered, such as in autism and epilepsy. The zebrafish model offers an unparalleled opportunity to ask whether neurons perform re- scaling in vivo because of its transparency for optical approaches and the wealth of molecular-genetic and physiological tools available. In this proposal, I examine homeostasis of inhibitory glycinergic synaptic proteins in vivo and test the possible roles of neuronal activity, circadian rhythms, and behavioral sleep/wake states in the regulation of synaptic re-scaling. Glycine plays the dominant inhibitory role in the spinal cord and few studies have addressed homeostatic regulation of glycinergic synapses. Aim 1 seeks to determine how spinal cord motoneurons alter the relative rates of accumulation and removal of synaptic proteins from glycinergic synapses during the day and night. These rates will be measured in spinal motoneurons expressing the convertible protein dendra fused with glycinergic proteins by quantifying the rate of removal of converted protein and accumulation of non-converted protein during day and night. Aim 2 seeks to physiologically test changes in the strength of glycinergic synapses between day and night. The CatCH-eYFP channelrhodopsin variant will be expressed specifically in a class of glycinergic interneurons in the spinal cord an used to directly measure the mean amplitude of postsynaptic inhibitory currents in postsynaptic motoneurons. Aim 3 will determine whether the patterns of regulation observed in the first two Aims are regulated through circadian rhythms, sleep/wake states, or neuronal activity. In order to determine if circadian rhythms regulate synaptic re-scaling, the dynamics of dendra-tagged synaptic proteins will be measured (as in Aim 1) during the day and during sleep deprivation at night. A pattern of regulation identical to that observed in the previous Aims, would suggest that circadian rhythms play a regulatory role, because both neuronal activity and wake state have been altered by sleep deprivation. If this is not the case, the role of neuronal activity will be tested by perturbing the activity of single motoneurons and measuring synaptic dynamics. An inward rectifying potassium channel will be expressed in motoneurons to reduce activity and the CatCH construct expressed to increase activity. A lack of effect of activity would suggest a possible role for other sleep related changes in the process, such as changes in neuromodulatory levels. These experiments will provide data on re-scaling in the glycinergic synapse as a function of behavioral state and increase our understanding of how re-scaling mechanisms contribute to the function and stability of neural circuits in a living animal.